Correlation of Aflatoxin Contamination in Peanuts with Rhizosphere Microorganisms and Its Comprehensive Control

Suyan GAO Shoumin JIA Litian ZHANG Yi ZHANG Haitao Zhao Aili XIANG Biao QI Lixue DONG

Abstract Aflatoxin contamination is an important hazard factor affecting the quality safety and food safety of peanut production， which seriously threatens peoples health and safety. In this paper， the background and harmful causes of aflatoxin contamination were analyzed in detail， with attention focusing on its control measures， which has certain guiding significance for the comprehensive control of aflatoxin contamination.

Key words Peanut; Aflatoxin contamination; Comprehensive control measures

Peanut is one of the major oil crops in the world， widely distributed in developing countries such as Asia， Africa， South America and some developed countries and regions such as the United States， Canada， and Australia. The peanut industry plays an important role in world agricultural production and trade. In 2013， the worlds total output was 41 million t， and the planting area was 24 million hm2， ranking second only to rapeseed in the oil industry. China is a major producer of peanuts in the world with an annual planting area of 5 million hm2， accounting for 25% of the total planted area in the world， ranking second， while the yield reaches 16 million t， accounting for 40% of the worlds total， ranking first. Peanut plays a major part in the export trade of China. As the worlds largest exporter of peanuts， China has an annual export volume of 700 000 t， accounting for about 40% of the worlds peanut trade volume[1]. The sustainable and healthy development of peanut planting industry， including China， is an important guarantee for the smooth development of the worlds peanutrelated industries. It is also of great significance for promoting the worlds economic development and maintaining social stability.

Peanut is a kind of agricultural product that is susceptible to aflatoxin contamination. Peanut is highly susceptible to toxin contamination caused by mold such as Aspergillus flavus during its growth， storage and transportation. Aflatoxins （AFT） are a kind of secondary metabolites produced by Aspergillus fungi such as A. flavus and A. parasiticus， which have acute and chronic toxicity， carcinogenicity and teratogenicity. AFT B1 is 10 times more toxic than potassium cyanide， 68 times that of arsenic， so it has been listed as a primary carcinogen by the World Health Organization （WHO） for its great harm to human health[2]. AFT contamination is a worldwide problem， and it is the most important risk factor for the safe consumption and export of agricultural products such as grain and oil， and has received extensive attention. Therefore， it is of important significance for the healthy development of peanut industry to take prevention and control measures for AFT contamination.

Status of AFT Contamination in Peanuts at Home and Abroad

Status of AFT contamination in peanuts in China

With the increase of peanut planting area in China， various diseases have been aggravated， especially the AFT contamination， which has become worse and worse year by year， posing a serious threat to the safety of food， oil and agricultural products and food safety in China. AFT is a kind of secondary metabolites produced by A. flavus and A. parasiticus with acute and chronic toxicity， mutagenicity， carcinogenicity and teratogenicity. With great toxicity， it has been listed as a primary carcinogen by the WHO[2]. On the other hand， peanut is one of the most vulnerable crops to AFT contamination. Peanuts may contaminate A. flavus and produce toxins from every stage of production to consumption， which can cause great harm to human health. China is a country with serious AFT contamination in peanut. Take the shelled peanuts as an example： Ding conducted a sample inspection of peanuts in 2009 and 2010. The results showed that the detection rate of AFTB in peanuts was 21.7% nationwide with an average content 6.82 g/kg and maximum value of 743.41 g/kg; the average content of total aflatoxins was 40.34 g/kg， and the maximum value was 5 271.41 g/kg， which greatly exceeded the national limit standard of 20 g/kg in China[3]. Gao et al. investigated and analyzed AFT contamination in the peanut producing areas in Jilin， Henan， Hubei， Sichuan， Guangdong， Guangxi， finding that AFT contamination was common in the 6 regions， and the contamination rate was close to 60%. Moreover， the contamination degree in the 6 regions was high， with the average concentration of 91.74 g/kg[4]. The detection rate of AFT in peanut in Taiwan was 7.8% with the average concentration of 14.9 g/kg， and the AFT standard limit set by Taiwan was 15 g/kg[5]. Another research showed that the AFT contamination was not only quite common in peanuts and their products in China， but also showed great differences in different regions. In general， AFT contamination became more serious from the north to the south of China. The contamination was relatively milder in the peanut producing areas in central China， while the AFT contamination was generally heavier in the Yangtze River Basin and the southern regions of China[6-8]. In north China， AFT contamination mainly occurs in the process of peanut harvesting， storage and shelling， while in the south， the contamination of AFT mainly occurs in the production process. The main reason lies in the difference of temperature between the north and south of China. The high temperature and high humidity， as well as the long duration of rainy season in the south makes it more favorable for the growth of A. flavus on peanuts， thereby producing AFT.

Status of AFT contamination in other countries

AFT contamination in peanuts is also a common problem all over the world. The occurrence rate of AFT in peanuts is 45% in India is 45%， and the content range is 5-833 g/kg; the AFT occurrence rate in Brazil is 51% with the content ranging from 43 to 1 099 g/kg; the rate o in Congo Democratic Republic is 72% with the content range of 1.5-937 g/kg. Once contaminated with A. flavus and its toxins， peanuts would suffer from largescale reduction of output， which can cause huge economic losses to producers and exporting countries， as well as pose serious threats to the health of humans and animals. According to statistics， the annual loss of trade caused by AFT contamination is as high as several billion dollars in the world， in which the annual loss in Africa is about 450 million dollars， and the annual loss of the American peanut industry is more than 20 million dollars[9-10].

As a global problem， AFT contamination in peanuts is the biggest and most important risk factor for peanut safe consumption and export， and it has received wide attention. Countries and international organizations around the world have made strict AFT limits on the production， export and import of peanuts and their products. However， it is a worldwide problem to control the AFT in peanuts， and there is still no fundamental solution.

Correlation between AFT Contamination in Peanuts and Rhizosphere Microorganisms

Aflatoxins are produced from the interaction between A. flavus/A. parasiticus， the host （matrix） and microecological environmental conditions. The occurrence of plant diseases is the result of plant microecological imbalance. Once the balance is broken， it will lead to diseases[11-13]. The infection of A. flavus to peanut plants can lead to root microecological imbalance. As a result， toxin contamination occurs. Therefore， it is of important significance for the precise prediction and scientific control of AFT contamination in peanuts as well as for promoting the healthy development of peanut industry to explore the microbial community structure related to AFT infection and its toxins contamination in peanut rhizospheres and roots， and to clarify their related functional specificities.

Effect of continuous cropping on microbial community in peanut rhizosphere

Many scholars have studied the effects of rhizosphere microbial changes on disease in longterm continuous cropping of peanut fields in China. Bulgarelli et al. and Zhang et al. studied the effects of continuous cropping of peanuts and maize on microbial population and soil nutrient status in peanut soil. It was found that continuous cropping could significantly increase the number of peanut rhizosphere bacteria， and the function of continuous soil microbial population could be improved， significantly reducing the degree of fungal damage including A. flavus[12，14]. Tang et al. found that the continuous cropping of peanuts with cassava could significantly increase the number and ecological diversity of fungi， bacteria， actinomycetes and total microorganisms in peanut rhizosphere soil， promoting the resistance of plants to diseases[15]. Sun Xiushan et al. studied the microbial diversity in the rhizosphere soil of peanut after 5 years of continuous cropping， and the results showed that the number of bacteria in rhizosphere soil increased with continuous cropping years after continuous cropping for many years. However， the number increased after continuous cropping for a certain number of years， and the number of fungi and actinomycetes presented overall increasing trends[16].

Effects of different managements on microbial community in peanut rhizosphere

Zhang et al. found that the application of different herbicides in peanut fields significantly inhibited symbiotic bacteria such as rhizosphere nitrogenfixing rhizobia and increased the occurrence of fungal diseases including A. flavus[17]. Lin et al. investigated the effects of peanut planting methods on soil microbial ecological environment， finding that different planting methods had significant impacts on the microecological environment and peanut yield in peanut rhizosphere soil[18]. There are also studies at home and abroad finding that the total number of rhizosphere soil microorganisms was greater in infected peanut plants than in healthy peanut plants. The change of microbial community structure in peanut rhizosphere， especially the increase in the number of fungi， is the most obvious feature of fungal diseases.

The above studies indicate that there is a close relationship between A. flavus and microbial community structure in soil， which will play an important role in promoting the comprehensive prevention and control of AFT contamination.

Control Measures for AFT Contamination in Peanuts

Control measures based on agricultural control and supplemented by chemical control

More than 65% of the aflatoxins in peanut agricultural products are infected in the middle and late stages of peanut growth. Therefore， a good control against the infection of A. flavus to peanuts in the growth period， especially the mature period， can significantly reduce the level of AFT contamination in peanut products[19-22]. At present， the control of AFT infection during growth is mainly based on agricultural control[23-27]， and the varieties resistant to （resistant） diseases are selected， supplemented by chemical control. However， it is difficult to implement the rotation method in the peanut production regions in China， which are more effective among the agricultural control measures. On the other hand， there is a lack of varieties with high resistances to the infection of A. flavus in production[28]. As a result， the effect of chemical pesticides is not obvious， while the longterm， largescale and repeated use of chemical pesticides has caused serious environmental pollution and other problems.

Biological control

The control of AFT contamination should lay the focus on the source. Biological control has received extensive attention and attention in recent years due to green control and ecological safety. The main biocontrol bacteria currently studied are Trichoderma spp.， Bacillus spp.， Actinomycete spp. and Lactobacillus spp.[21，24]. Guan et al. studied the use of metabolites of Bacillus spp.to inhibit AFT contamination in peanuts， which had achieved good control effects， and the indoor experiment showed that the inhibition rate of toxins was up to 80%[29]. Wei et al. reported the use of Trichoderma and its metabolites to control the infection of A. flavus to peanut seeds， as well as its effect on seed vigor. The results showed that biocontrol bacteria treatment could significantly reduce the infection of A. flavus to seeds， enhancing the germination vigor of seeds.[30]. Wei et al. used Bacillus spp. to control AFT infection and the production of toxins in peanuts， and the effect was remarkable. Yan et al. investigated the inhibitory effects on A. flavus by using the plants which produced no toxic A. flavus， which achieved remarkable controlling effects in potted experiments and field trials[31]. Kong et al. studied the inhibitory effect of A. flavus on peanut kernels by using a strain of marine B. megaterium， and the results showed that metabolites had a higher inhibitory effect on A. flavus hyphae[32].

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